This inventive method and apparatus is used for the remote sensing of digitally encoded data through voice-grade telephone lines, and more specifically describes a central computer and associated circuits connected to user sites by telephone lines to detect switch closures at the remote sites.
Equipment at a central location capable of automatically sensing digitally encoded information at a plurality of remote sites would be valuable to the utility companies and others who have a requirement for periodically updating numerical data from remote sites. The present practice at the utility companies is to have meter readers visit each building every month to record the information. At the central office these records are transcribed into machine readable form and entered into a computer.
An improved method requires digital encoding apparatus at the users site and a recording instrument, usually comprising a tape recorder, carried by the meter reader. When the portable recording apparatus is coupled to the encoding apparatus at the users site, the customer ID number and the current meter reading is automatically recorded on magnetic tape in machine readable form. Using this method, the transcription step from paper copy to machine readable form at the central location is avoided, but the meter reader still must visit each user's site.
The problem is compounded when the data required is to be updated daily or hourly. This would be necessary, for instance, in a large factory where power usage (water usage, etc.) is to be monitored for the purpose of conserving energy or avoiding high peak usage at specific times of the day. In this example, an automatic system would be advantageous since it would avoid the requirement of meter reading personnel and also allow real time control of the power (water) through the use of the computerized reading apparatus to generate usage commands to control the utility being measured. Of course, dedicated phone lines or hard wiring of any kind to the central location will accomplish the function, but at high cost. A method of accomplishing the reading of this digital data through the use of telephone lines already installed without interruption of the telephone service would accomplish the result at low cost.
An improved method and apparatus for using phone lines to monitor remote switches is described in U.S. Pat. No. 3,922,490, Alarm and Utility Meter Reading System Employing Telephone Lines, by Charles D. Pettis, and is incorporated herein by reference. This patent describes apparatus comprising a series of resistances in parallel or series with the telephone at the users location and a set of switches, each of which, when closed, shorts out one resistor. With the use of a particular set of resistance values, sensors at the central location can determine which switches are closed. By attaching each switch to a dial (usually the least significant) of any utility monitoring device, by initializing the central computer meter data, and by using each sensed switch closure to update the data, an automatic system can be implemented.
This system may be usable over short distances. For example, the electricity usage in one building may be accurately monitored. However, noise problems become apparent at longer distances. For instance, the noise generated on telephone lines connecting residential houses over several square miles exceeds the signal value by a factor of five or more. Since the telephone equipment cannot be modified a method of reading small digital signals in a high ambient noise background is required.